The external silicon (Si) geochemical cycle is closely linked to the carbon cycle and therefore has been increasingly scrutinized in the recent years by the scientific community, tough mostly in oceanic studies. At the Earth surface, Si is both tightened chemically to weathering and biologically to aquatic (diatoms, sponges...) and terrestrial life (phytoliths in vascular plants). Si is also very sensitive to anthropogenic changes such as dams building, deforestation and eutrophication. The new silicon isotopic proxy, d29Si or d30Si, has demonstrated its great potentialities to quantify and qualify biogeochemical processes affecting Si cycle. However, due to analytical difficulties, recently overcome by the spread of MC-ICP-MS instruments, there are only few data published to date, especially for the continental sphere: 8 data for the rivers worldwide and 4 for phytoliths (in 2003). Yet these rivers control the global oceanic input and their geographical and seasonal isotopic variations remain to be assessed. Due to its huge annual discharge and its water chemistry, the Congo River is the second largest dissolved Si supplier to the ocean. It is also running through distinct climatic environments and vegetations (wet tropical, highlands, savannah...), which behave differently in terms of erosion and weathering processes.
The aim of this project is to quantify the relative contribution of diatom and phytolith fractions in the continental Si cycle and to constrain the paleo-, seasonal and spatial variations of the isotopic Si signatures in the Congo River Basin. In particular, by using this approach, we intend to decipher the respective role of weathering, biogenic and anthropogenic processes in generating the observed isotopic signatures and Si contents.